Jan. 7, 1918 Effect of Time of Digestion on Hydrolysis of Casein 



The results on humin represent the humin in solution and precipitated 

 by calcium-hydrate suspension. The high results obtained by Grindley, 

 Slater, et al. and Hart and Sure for humin nitrogen were made to include 

 the total nitrogen in the insoluble residue and also the humin in solution. 



It has been the experience of the writer that in cases where considerable 

 insoluble residue was included in the total volume of the hydrolyte, great 

 difficulty was met with in obtaining uniform aliquots for the total 

 nitrogen in the solution and also for the aliquot for determination. This 

 difficulty is avoided by filtering out and washing the insoluble residue. 

 Then, too, the question arises, Is it fair to consider the nitrogen remaining 

 in the insoluble residue as humin nitrogen? 



The results for arginin show no serious loss in any of the determina- 

 tions, and the minimum result obtained is only 1.5 per cent below Van 

 Slyke's result for arginin on casein alone. 



The histidin results are practically the same for the two 12-hour 

 digestions, on casein alone and on the casein-starch mixture. In the 

 15-hour casein-starch digestion the result for histidin is 0.61 per cent 

 above that reported in Van Slyke's analysis. In the 24- and 48-hour 

 digestions there is a loss in histidin nitrogen of considerably more than 

 50 per cent of that found in the 15-hour digestion. Hence, the results 

 for histidin in the two last experiments are very significant, indicating 

 that long periods of digestion of starch and casein bring about a redistri- 

 bution of the nitrogen in this group. It is to be borne in mind that Hart 

 and Sure^ obtained similar results on lysin. These writers also report 

 7.30 per cent as a average for histidin determinations in their experiment. 



There is a diminution in the cystin nitrogen of more than 50 per cent in 

 the 24- and 48-hour digestions. Hart and Sure state that their results for 

 cystin were so low that they reported the results obtained by Van Slyke 

 instead. 



The results for lysin agree well in the 12- and 15-hour experiments. In 

 the 24- and 48-hour experiments the results for lysin are high. Lysin 

 nitrogen is obtained by deducting the sum of histidin, arginin, and cystin 

 nitrogen from the total nitrogen in the bases; therefore any diminution 

 in the nitrogen content of either histidin, arginin, or cystin will increase 

 the results for lysin nitrogen correspondingly. 



There is no marked difference between the results obtained in all the 

 experiments for the amino- and nonamino-nitrogen content in the 

 filtrates from the bases. 



In the footings of the different analyses it is to be noted that the 

 1 2 -hour digestions give footings more than 2.5 per cent over 100. In 

 the 15-hour digestion the footing is good, while in the 24- and 48-hour 

 digestions the footings are 2.75 per cent less than 100, thus indicating 

 that the 12-hour experiments were probably not completely hydrolyzed; 



• Hart, E. B., and Sure, Barnett. Op. cit. 



